The objective of the proposed research is to better understand the biochemical mechanism(s) by which protein turnover is regulated in skeletal muscle and the role played by the nervous system. It is known that muscle use leads to cell growth while disuse by denervation or immobilization leads to muscle atrophy. These alterations are accompanied by changes in the rate of uptake of nutrients, and by changes in the rates of protein synthesis and protein degradation in the muscle cells. This project represents the continuation of our earlier studies using differentiated skeletal myotubes in tissue culture to study the regulation of protein turnover. Passive stretch of these uninnervated cells leads to a reversal of many of the changes seen on denervation; thus, stretch stimulates amino acid transport and protein synthesis, and inhibits protein degradation. These changes are accompanied by activation of the plasma membrane sodium pump which helps to regulate intracellular ion levels. We believe that intracellular cations play an important role in the regulation of protein turnover in skeletal muscle. The proposed work will involve measurement of cationic fluxes and equilibrium concentrations in tissue-cultured skeletal myotubes when the cells are passively stretched, stretched and relaxed, and when activity of the Na pump is altered by other methods. These changes will be compared to changes in total protein synthesis and degradation, and to the synthesis and degradation of several muscle-specific proteins-myosin heavy chains and acetylcholine receptors. These studies should lead to a better understanding of the neurotrophic effect of nerve on muscle.